Recently, polymers that are prepared by using biomasses such as plants as raw material are attracting attention in a trend toward establishment of a resource-circulating society. In particular, polylactic acid (hereinafter, referred to as “PLA”) was shown to have favorable properties as a polymer from biomass raw material.
Lactic acid, the raw material for PLA, is produced by fermentation of microbes, generally called lactic bacteria, represented by those of Lactobacillus species and Lactococcus species. Production of lactic acid by using lactic bacteria is superior in lactic acid yield from sugar and lactic acid production rate. However, the lactic acid obtained is a mixture of L- and D-lactic acids. Thus, there is a problem in optical purity. High optical purity is demanded for the lactic acid for use in production of PLA.
There were several attempts to produce high-optical purity lactic acid. For example, production of L- and D-lactic acids by using a transformant yeast strain was studied (for example, JP 2001-516584 A, JP 2003-093060 A, JP 2005-137306 A and N. Ishida et al., J. Biosci. Bioeng., 101(2), pp. 172-177, 2006). Yeasts do not have ability to produce lactic acid inherently. It was reported in these literatures that high-optical purity lactic acid could be obtained by introducing a gene coding lactate dehydrogenase, which converts pyruvic acid into lactic acid, into yeast by genetic recombination technology. On the other hand, the lactic acid yield and the lactic acid production rate during lactic acid production by yeast are lower, compared to those by lactic bacteria. It is thus needed to improve both lactic acid yield and lactic acid production rate for production of lactic acid by using a yeast at low cost.
For improvement of the lactic acid yield and the lactic acid production rate at the same time, a method of culturing a yeast strain having a lactic acid-producing ability while the fermentation solution is filtered through a separation membrane was developed (see, for example, WO 2007/97260). However, even if the method was used, it caused a problem that both the lactic acid yield and the lactic acid production rate declined during fermentation.
It could therefore be helpful to provide a gene expressing cassette coding lactate dehydrogenase that is needed for prevention of deterioration in lactic acid yield and lactic acid production rate in continuous culture with simultaneous filtration of a yeast strain having a lactic acid-producing ability, which achieves high optical purity, high lactic acid yield and high lactic acid production rate simultaneously, a yeast strain having the cassette and a method of producing lactic acid by culturing the yeast strain.